One good reason for creating a new Domain is if your current network contains lots of account and resource domains and you want to consoladate multiple domains to just a few or one. Since NT wasn't capable of handling tons of users, this was common practice to do. 2000 works well as a single domain and new features (organizational units) provide excellent ability to replace resource and account domains in a single domain structure. From the size you describe, you most likely do not fall under this condition. The other good reason is if you want a clean environment with all new user accounts, security settings, etc. Before the migration, you give the access control lists to your objects permissions for both your present domain and the new domain you are creating, then migrate users over to their new accounts. With your size, this wouldn't be too bad and it makes it more seamless to the users (they only have to log into the new domain name, the old domain is there for backup till deleted).
You are concerned with retaining old settings... this suggests migrating your old domain into 2000. The upgrade process can be done on your old servers or you can use a new server as your target. If just upgrading existing servers, the process is fairly straight forward. If you intend on using a new server, you have a few extra steps...
On the new server, install NT server as a BDC. This allows it to pick up all your SIDs from the PDC. Now, up the new server to being the PDC. If you were upgrading just the old servers, you would start with the PDC.
Upgrade the PDC to 2000. At the completion of the upgrade process, you promote the PDC to a domain controller (the command prompt program dcpromo will do this, or you can use the "configure your server" from the administrator utilities). Pick the mixed environment, you're not fully 2000 yet. You will be installing DNS during this process and picking a DNS name for your domain (NT only used netbios and WINS). Your settings are retained... most importantly, your SIDs for the user accounts are retained. The new 2000 server will now interact with your remaining NT servers like it was one itself. You can add new 2000 servers, upgrade other NT servers (or decommission them) now.
As a precaution, do full backups before upgrading, and you can also remove an existing BDC from the network during the upgrade process to expedite a recovery if necessary. If you are using RAS on an NT server, it will not function with 2000 server unless the server is added to the pre 2000 compatable security group in 2000. WINs databases will need upgrade or delete them to be recreated in 2000.
Once all your DC's (NT servers that are not domain controllers can be done later) are on 2000, you can switch to native mode and take advantage of all of 2000's features (i.e. new global security groups, RAS policies). If you look in the help for 2000 as well as the tools on the CD, you will find excellent documentation on upgrading to 2000 as well as some nice utilities to help you.